Dechlorinating water for discus: products, dosing and mistakes

In short

• Tap water often contains chloramine (NH₂Cl) — it doesn't evaporate and needs a chemical dechlorinator.
• Seachem Prime is the standard: it neutralizes Cl, chloramine, NH₃, NO₂ and heavy metals.
• Prime dose: 1 ml / 40L of water (1 cap = 5 ml = 200L).
• A double Prime dose (1 ml / 20L) in an emergency or with high NH₃ water.
• Don't skip the dechlorinator even for small changes — chlorine/chloramine burns the gills.
• Add the dechlorinator to the fresh water, NOT into the tank directly (exception: an NH₃ emergency).

What you'll learn

Tap water is treated with chlorine (Cl₂) and often chloramine (NH₂Cl) — essential for public health, but lethal for fish even in small concentrations. Dechlorination is not optional — without it, discus gills burn, filter bacteria die, and the fish go into respiratory shock within hours. This guide compares the four main products and shows you how to use them correctly.

Chlorine vs chloramine — the critical difference

⚠️ Many water utilities use chloramine because it's more durable in the pipes. 24h of aeration does NOT work — a chemical dechlorinator is mandatory. Check what your local utility uses.

Top 4 dechlorinators — comparison

1. Seachem Prime ⭐⭐⭐⭐⭐ (recommended)

Why it's the standard:

• The only one that temporarily neutralizes NH₃ and NO₂ — vital in an emergency
• ~5× concentration — a 500ml bottle lasts 1–2 years
• Compatible with most medications (do NOT combine with peroxide)

Note: a strong sulfurous smell — that's normal.

2. Tetra AquaSafe ⭐⭐⭐⭐

Pro: B vitamins + aloe vera (slime-coat protection), pleasant smell. Con: a larger dose → higher cost per liter than Prime.

3. JBL Biotopol ⭐⭐⭐⭐

Pro: widely available, consistent quality. Con: doesn't neutralize NH₃/NO₂.

4. Sera Aquatan ⭐⭐⭐

Pro: acceptable price/efficiency ratio. Con: no NH₃/NO₂ action.

Cost per liter treated

Prime works out far cheaper per liter treated than the others because of its high concentration — a small bottle treats a very large volume. Exact prices vary by region, so compare locally; Prime's value advantage holds nearly everywhere.

How to use a dechlorinator correctly

Step 1 — In the change water (recommended)

1. Prepare the fresh water in a separate container
2. Add the dechlorinator TO the container (not to the tank)
3. Stir well for 30 seconds
4. Air pump on for 5 minutes (oxygenation)
5. Add to the tank slowly

Step 2 — In the tank (emergency only)

1. Only if you forgot the dechlorinator after already adding the water
2. Add a double dose for the whole tank volume
3. Turn on extra aeration
4. Test NH₃ + Cl after 2 hours

Doses by scenario

Common mistakes

❌ "I leave the water aerating for 24h, that's enough" — false for chloramine ❌ Adding dechlorinator straight onto the fish — stress + local chemical swings ❌ Too small a dose "to save money" → burned gills ❌ Repeated very large doses → temporary biofilter suppression (rare) ❌ Combining with medications without reading the labels — Prime inactivates some treatments ❌ Using boiled water "to remove chlorine" — it concentrates metals, zero oxygen

What a dechlorinator does NOT do

• ❌ Doesn't remove nitrate — change the water
• ❌ Doesn't remove phosphate — use a resin or plants
• ❌ Doesn't adjust pH — use peat, baking soda
• ❌ Doesn't add bacteria — Seachem Stability separately
• ❌ Doesn't sterilize water — UV for that

Useful combinations

Dechlorination checklist

• ☐ A separate container for preparing change water
• ☐ Exact dose measured with a syringe (NOT "by eye")
• ☐ Stir 30 sec after adding the dechlorinator
• ☐ Aerate 5 min before adding to the tank
• ☐ Identical temperature ±0.5°C / ±1°F
• ☐ Bottle stored sealed, at room temperature
• ☐ Backup Prime at home (NH₃ emergency)

What to do next

📚 Learn more:

• Ammonia and nitrite — use Prime in an emergency
• How often to change the water
• Mixing RO + tap water

🛒 Buy:

• Verified breeders and trusted stores for Seachem Prime and chlorine/chloramine test kits.